Spatial light modulators used in sequential color display systems are capable of projecting image details from media sources such as HDTV, DVD, and DVI. Viewers evaluate display systems based on many criteria such as image size, resolution, contrast ratio, color purity, and brightness. Image brightness is a particularly important metric in many display markets since the available brightness can limit the image size of a projected image and controls how well the image can be seen in venues having high levels of ambient light. Projection display designers increase the brightness of a given projection display by increasing the light source used to form the image. Increasing the light source, however, also increases the cost, size, and weight of the display system. Additionally, larger light sources generate additional heat that must be dissipated by the display.
Many other factors affect the brightness of the images produced by the display system. One of the major factors is the number of modulators used to modulate the light used to produce the image. Display systems that use a modulator with a very fast response time, such as the digital micromirror device (DMD®), can use a single modulator to create a full color image. Other display systems use three modulators, such as liquid crystal display (LCD) panels or DMDs, to create a full color image. The disadvantage of the single-modulator sequential color display system is its low image brightness. Because the white light source is time-divided into three or more primary color light beams, most of the light at any given time is not used. For example, when the blue primary color image is being formed, the green and red output of the white light source are filtered out of the light beam and “dumped.” Thus, a sequential color display system, while generally less expensive than the three-modulator display system, makes relatively inefficient use of the light produced by the light source.
In addition to reducing the brightness of the image produced by the display system, discarding or “dumping” portions of light creates other problems for the display system. For example, the light filtered out of the light beam generally becomes stray light that the display system must control to prevent the filtered light from reaching the image plane and degrading the contrast of the displayed image. Additionally, the filtered light is generally converted to heat, which must be dissipated by using large fans that increase the noise produced by the display system and increase the size of the display system.